A capacitor is a device for obtaining an electric capacitance and is a kind of an electric storage device along with a storage battery. Recently, the usage of an ultracapacitor (or a super-capacitor) such as a metal oxide pseudo-capacitor, a hybrid-capacitor obtained by combining a storage battery and an electric double layer capacitor, is spread as an electric power source for an industrial field such as an electric automobile, a hybrid automobile, an electric power source for the solar energy, an uninterruptible power supply (UPS), and the like.
The ultracapacitor is an electric energy storage device for storing an electric energy within active carbon or metal oxide by means of a physical reaction or an electrochemical reaction having a rapid reaction speed. The ultracapacitor has a better cycle characteristic and a better temperature characteristic when comparing with those of the conventional storage batteries.
FIG. 1 is a graph illustrating a cycle characteristic of an electric double layer capacitor, which is one of an ultracapacitor and FIG. 2 is a graph illustrating a temperature characteristic of an electric double layer capacitor, which is one of an ultracapacitor. As concretely confirmed from FIGS. 1 & 2, the ultracapacitor has a cycle lifetime of about 100,000 times or over and so the ultracapacitor can be regarded as having almost permanent lifetime. In addition, the ultracapacitor has a good temperature characteristic within a wide temperature range of from about −45° C. to about 60° C. Therefore, this capacitor can be used as a starting device of an automobile engine at a low temperature.
However, the price of the ultracapacitor is about 20,000 won/Wh, while that of the lead acid battery is about 60 won/Wh. Therefore, even though the ultracapacitor has good characteristics, the spread of the ultracapacitor is slow. In contrast, since the lifetime of the storage battery is about 2-3 years, a large amount of keeping and repairing cost is required. In particular, the storage battery is widely used as the electric power source of the solar battery for the indicating light, especially used on a road or out in the fields until now. However, the application of the ultracapacitor is increasingly spread due to the excessive cost for keeping and repairing of the storage battery.
In the meantime, researches on an ultracapacitor having an improved energy density, a prolonged cycle lifetime and a low manufacturing cost are actively executed. For example, a hybrid capacitor including one electrode formed by using active carbon as an active material, which is generally used for the manufacture of the electric double layer capacitor and including the other electrode formed by using lithium intercalated graphite, which is generally used for a lithium ion battery or by using a material having a large capacitance such as Li4Ti5O2, has been developed. Through the hybrid capacitor, the capacitance efficiency of the active carbon is increased and a charging/discharging depth of a secondary battery during charging/discharging is decreased. In addition, the problem concerning the cycle lifetime, which is a disadvantage of the secondary battery can be solved. However, obtaining a satisfactory result concerning both characteristics of the capacitor and the secondary battery using one electrolyte is not an easy task.
Generally, the secondary battery is inexpensive and has a high energy density when considering the capacitance thereof However, the characteristic change according to the temperature is large because an electro-chemical reaction is utilized for storing electric energy. In addition, since the electrode becomes deteriorated according to the progress of the charging/discharging cycle, the cycle lifetime is relatively short. This will be described referring to FIG. 3.
FIG. 3 is a graph illustrating a temperature characteristic of a lead acid battery. As illustrated in FIG. 3, the most widely used secondary battery, the lead acid battery exhibits only 62% of the capacity at 0° C. and 33% of the capacity at −25° C. with reference to the capacity at 25° C. Accordingly, the secondary battery including the lead acid battery has an inferior temperature characteristic, particularly an inferior low temperature characteristic and has a cycle lifetime of about no more than 300 cycles.
When the lead acid battery is employed for an automobile, the starting of the automobile at a low temperature of subzero temperature frequently fails due to the bad low temperature characteristic. When the lead acid battery is employed for the solar system, a problem concerning the operation of a system including the lead acid battery is generated due to the lowering of the capacity and the deterioration of the charging performance. When the lead acid battery is employed for the solar system, a large lead acid battery having a large capacity can be used to solve the above-described problem. Then, the operating time of the system can be prolonged under the circumstance of an incomplete charging. However, the charging performance at a low temperature is further deteriorated after discharging because the capacity is large. This phenomenon is not limited to the lead acid battery but is a general characteristic of the secondary battery.
In order to solve this problem, a new system obtained by combining a capacitor and a secondary battery in parallel is widely used. In this system, the lifetime can be increased and the power characteristic can be improved by lightening the load of the secondary battery by utilizing the good power characteristic of the capacitor by entrusting the peak power to the capacitor.
According to this system, the lifetime of the secondary battery can be increased and the power characteristic can be improved. However, the cost is not reduced when considering the capacity. In addition, when the electric energy storage devices including the secondary battery and the capacitor are connected in series, generally, the same type of secondary battery or capacitor having similar capacities are used for reducing the voltage deviation between cells.